AIDS is a viral disease, and clinical progression is associated with increased viral burden. Accurate assessment of HIV-1 level in vivo will be critical for monitoring treatment efficacy. Replicative activity of HIV-1 in patients can now be determined to an extent by routine immunoassay for p24 core protein in serum and by virus isolation. both of these virologic tests have been used in our laboratory for many years, and this expertise is well documented by our publications since 1984. In addition, we were the first to develop quantitative cultures which can determine the infectious titers of HIV-1 in the plasma and peripheral blood mononuclear cells of patients. Furthermore, we have utilized these end-point-dilution assays to demonstrate the presence or absence of in vivo efficacy of antiviral agents. In the past two years, quantitation of HIV-1 DNA in clinical samples by PCR methods has also been achieved in our laboratory. This quantitative PCR technique has permitted the detection of fluctuations in HIV-1 load in the patients encountered by us. We now propose to perform p24 antigen assays and HIV-1 cultures as a routine service for the clinical trials conducted at the NYU ACTG. In select clinical situations, quantitative culture and PCR tests will also be done to determine the antiviral activity of new therapeutic agents.